Display omitted •Activated carbon supported monodispersed Co3O4 nanoparticles (Co3O4/AC) were prepared via a facile deposition method.•The presence of Co-OH+ on Co3O4/AC is highly efficient in PMS activation for gaseous toluene degradation.•SO4− play a dominant role for toluene degradation, HO derived from PMS activation can improve carbon mineralization.•Few gaseous intermediates were discharged during reaction process. Gas-phase VOCs decomposition generally produces intermediates and causes secondary air pollution. To avoid this issue, we proposed a novel method for a typical gaseous VOC (toluene) degradation via catalytic activation of peroxymonosulfate (PMS) in the liquid phase. Herein, activated carbon supported monodispersed Co3O4 nanoparticles (Co3O4/AC) were prepared via a facile deposition method. It is highly efficient in PMS activation for toluene degradation due to the presence of Co-OH+ species and well dispersed Co3O4 on Co3O4/AC. A toluene removal efficiency of nearly 90% was maintained during the reaction, and few gaseous intermediates were discharged. Sulfate radical (SO4−) and hydroxyl radical (HO) derived from PMS activation played different roles during toluene oxidation and mineralization. Electron spin resonance (EPR) suggested that the generation of plentiful SO4− resulted in the superior toluene degradation, and the presence of HO can improve carbon mineralization. Radical quenching tests further confirmed that SO4− played a dominant role for toluene degradation, whereas the absence of HO inhibited the carbon mineralization. The toluene degradation pathway in the Co3O4/AC-PMS system was proposed based on the intermediates identified by GC–MS.